There have been a number of studies supporting the efficacy of spinal manipulation, the signature therapy of chiropractic care, for low back pain (LBP). Yet, there have been no large trials to determine optimal care in terms of the number and frequency of treatments, or duration of care. Our pilot study was the first trial to investigate a dose-response relationship between the number of treatments with spinal manipulation and relief of LBP. The results indicate a dose-response relationship and provide preliminary data in support of this R01 application. The purpose of our 5-year trial is to evaluate dose-response, efficacy across dose, and cost-effectiveness of spinal manipulation provided by chiropractors for the treatment of nonspecific, chronic low back pain of mechanical origin. This study is innovative in that it will investigate efficacy of manipulation across treatment dose, as well as control attention bias, patient expectation, and the effects of touching the patient. It is significant in that it will be the first full-scale trial evaluating the effects of the number of treatments with spinal manipulation on pain and disability outcomes. It will inform evidence- based treatment protocols, third-party payer reimbursement, and schedules of chiropractic care used in future trials on chronic LBP. The study of dose-response is a priority in the NCCAM strategic plan. The study protocol is designed to compare manipulation to a light massage across dose after controlling attention, number of visits with care rendered, and the laying on of hands. A total of 400 participants will be randomized to 4 treatment groups (n = 100/group). They will be allocated to dose (9 or 18 primary- intervention visits) and to primary intervention (spinal manipulation or light massage control). A brief, focused light massage will serve as the comparison therapy for the assessment of efficacy across dose. All participants will attend 18 sessions with a chiropractor: 3 visits per week for 6 weeks. The treatment protocols will be standardized for all visits: assessment (history and physical), preparatory hot pack, primary intervention (9 or 18 visits), and low-intensity pulsed ultrasound. Participants receiving only 9 sessions for a primary intervention will also have 9 attention control visits. The inclusion of low-intensity pulsed ultrasound will control the number of visits with some form of treatment. The laying on of hands will be controlled by extension of the manual spinal palpation assessment. The specific aims are: 1) to determine the effect of the number of treatments and the efficacy of spinal manipulation across dose levels on clinical outcomes for chronic LBP sufferers, and 2) determine the cost- effectiveness of spinal manipulation and the number of treatments for the care of chronic LBP.